Liquids such as oils, fuels and hydraulic fluids are very frequently used in environments in which they are liable to become contaminated, typically with particulate matter. Taking engine lubricating oils as an example, engine components over which the oil washes are subject to wear, creating small particles, often referred, to as “chips” or “fines”, which are entrained in the oil flow.
These particles, and other particulate debris, may indicate engine component wear and are factors in the deterioration of the condition of the oil and may also cause damage to other engine components if allowed to freely circulate with the lubricating oil flow. In-line collectors, such as filters and gauzes, are therefore used to collect the debris, the collectors being checked and emptied on a regular basis.
Collection and analysis of the particulate debris can also provide information about the condition of oil washed components of the engine. For example, an excessive amount of debris can indicate excessive wear of a component and thereby highlight a potential problem. By analysing the debris, in particular its composition, it is also possible to narrow down the number of components from which the debris might originate, making the task of identifying the faulty or problem component an easier one. In some cases debris may be indicative of a precursor to failure. For example, if bearing cage coating material is identified indicating that the cage is damaged, it will be some time before a bearing is likely to fail due to this damage.
However, the regular checking and emptying of the collectors, and the analysis of the collected debris, amount to a burdensome manual maintenance requirement that it would be desirable to avoid. Moreover, the checking and emptying of the collectors are intrusive processes, which must necessarily be undertaken when the engine is not operating.
In addition to the problems associated with contaminants, particulate or otherwise, liquids such as those discussed above, which often work in very harsh environments, tend to experience a gradual deterioration over time. This deterioration may be of the base liquid itself, for example a change in structure or composition, and/or a loss or reduction in the efficacy of intentional additives (liquid or particulate) to the liquid, for example rust inhibitors or friction reducing additives in oil, which breakdown over time. As with the detection of contaminants, the task of monitoring this deterioration, by sampling and analysing the oil or other liquid, is intrusive and time consuming.
EP 1191330 describes NMR techniques for the detection and analysis of anomalies in fluid systems. However, before discussing these techniques further, it is useful to give a brief overview of the relevant NMR theory.
Certain atomic nuclei possess angular momentum and the quantum property of “spin”. Because the nuclei also carry a charge, specifically a positive charge, there is a magnetic moment associated with this spin. When placed in a magnetic field, these nuclei, which might be referred to as the nuclear “magnets”, tend to align with the field direction. Only certain orientations are possible—two in the case of a spin ½ nucleus such as a proton.
The energy difference between the orientations of the nuclei (“Zeeman splitting”) depends linearly on the strength of the magnetic field B. Transitions between the two orientations can be induced when the frequency of an applied oscillating magnetic field (normally electromagnetic radiation such as a radio frequency (RF) signal), exactly matches the energy difference. This so called resonance condition, is defined by the Larmor equation:ω=γBwhere ω is the angular frequency of the oscillating magnetic field (electromagnetic radiation) and γ, referred to as the magnetogyric ratio, is a constant for a particular nuclear species.
Different nuclei have different values of γ and so resonate at different frequencies in a magnetic field of given strength. For example, at 11.7 T, resonant frequencies for the following nuclei are: 1H—500 MHz; 13C—125.7 MHz; 27Al—130.3 MHz; 29Si—99.3 MHz; 51V—131.5 MHz; 53Cr—28.3 MHz; 55Mn—123.3 MHz; 59Co—118.1 MHz, 95Mo—32.6 MHz; 107Ag—23.3 MHz and 183W—20.8 MHz.
The magnetic field B in the Larmor equation given above is the actual field strength at the nucleus and includes susceptibility effects arising from the bulk magnetic properties of the sample, local variations in these effects due to sample heterogeneity, and the screening effect of the electrons that surround the nucleus itself. Thus:B=B0(1+χ)where χ is the magnetic susceptibility and B0 is the applied magnetic field.
In traditional high resolution NMR, it is the contribution of the screening electrons to χ that gives the technique its power to analyse chemical structure: the same nucleus (e.g. 1H) will experience different magnetic fields depending on the chemical environment, so that chemically distinct nuclei resonate at slightly different frequencies. The range of these chemical shifts for any particular nucleus is, however, small: 0–10 ppm covers most 1H resonances of interest. In order to resolve them, the main applied magnetic field B0 must be maintained homogeneous over the sample volume. A few parts in 109 are commonplace and a few parts in 1010 achievable with spinning samples under ideal conditions.
However, the general premise on which the NMR techniques of EP 1191330 are based is that the NMR characteristics of a fluid system can be influenced by anomalies in the fluid system.
In particular, EP 1191330 proposes that such anomalies can be detected and analysed in two ways. The first way is termed “indirect detection” and involves analysing the influence the anomalies have on a signal from the fluid rather than analysing a signal from the anomalies themselves. This is particularly useful for the detection of inhomogeneities, such as particulates, in the fluid. The approach is possible where the particulates have a different magnetic susceptibility than the fluid, because they will then cause local non-uniformities in the magnetic field. This in turn modifies the NMR signal from the fluid, manifesting itself, for instance, in changes of line-width and/or position of the fluid resonance seen in the NMR frequency domain.
The second way is termed “direct detection” and, to the extent that the NMR signal that is detected and analysed does derive directly from a contaminant or additive in the fluid system, it is closer to traditional NMR techniques.
Direct detection can be used to detect inhomogeneities or dissolved species in the fluid.
According to EP 1191330, the above detection techniques can be performed using pulsed or continuous wave (CW) NMR spectroscopy.
Historically, CW NMR spectroscopy was the first to be developed (see e.g. N. Bloembergen, “Nuclear Magnetic Relaxation”, W. A. Benjamin, Inc., N.Y., 1961 and, E. R. Andrew, “Nuclear Magnetic Resonance”, Cambridge University Press, Cambridge, 1955). In CW NMR, a first, non-oscillating magnetic field of a predetermined field strength is created across a sample, which is also exposed to a second, oscillating magnetic field orthogonal to the first. The frequency of the oscillating magnetic field or the strength of the non-oscillating magnetic field is varied to sweep through the resonance condition and generate an NMR signal. For example, Bloembergen and Andrew both describe experimental arrangements in which a permanent magnet produces a “steady” non-oscillating magnetic field across the sample, and adjacent coils vary the strength of this steady field.
Modern NMR machines, however, mostly use pulsed NMR spectroscopy. These involve creating a first, non-oscillating magnetic field of a predetermined field strength across the sample, and intermittently exposing the sample to a second, oscillating magnetic field orthogonal to the first to generate an NMR signal. The relatively short pulse width (typically of the order of μs) of the intermittent field makes it possible to simultaneously detect a range of frequencies in the NMR signal. A significant advantage of pulsed NMR over CW NMR is that much greater signal-to-noise ratios can be achieved. Essentially, this is because the time between each pulse of the intermittent field is much shorter than each CW sweep through the resonance condition so that in a given period more useful information can be obtained with pulsed NMR than CW NMR (for a more detailed explanation see e.g. A. E. Derome, “Modern NMR Techniques for Chemistry Research”, Pergamon Press, 1987).
The trend in modern NMR spectroscopy is towards pulsed NMR with high non-oscillating field strengths and high resolutions. Largely this has been made possible by developments in the technology of superconducting magnets. It is difficult to modulate the magnitude of the field of such magnets, but so-called shim coils are used to locally increase the homogeneity of the field in the region of the sample under study. Most of these do so by producing magnetic fields varying in strength with distance from the magnetic centre (normally according to spherical harmonic distributions). However, the B0 shim coil produces a zeroth order, uniform field along the direction of the main B0 field that adds or subtracts from the main B0 field.
In contrast the NMR techniques described in EP 1191330 are suitable for performance at relatively low non-oscillating field strengths (e.g. 1.5 T or less). However, particularly at low non-oscillating field strengths (i.e. low resonant frequencies), a problem associated with the performance of NMR spectroscopy is that spurious resonances arising from the electrical or mechanical components of the NMR apparatus can interfere with the sample resonances, even to the extent of exceeding or swamping the sample resonances. Thus the spurious resonances can lead to low signal-to-noise-ratios.